The present invention relates generally to laser distance devices. More particularly, this invention relates to circuitry and methods associated with reducing walk error and accordingly increasing accuracy and resolution in time-of-flight laser distance devices.
Pulse laser time-of-flight distance measuring instruments are commercially available for use in hunting, golf, forestry and other wide-spread applications. Such instruments or laser rangefinders typically include: a collimated infrared pulsed laser at a nominal pulse repetition rate of several hundred Hertz or lower; a receiving photo detector; a circuit to recognize a return pulse from a target; a processing circuit to convert the lapsed time between the transmitted pulse and the received pulse; and a display to show the distance. An exemplary such laser rangefinder as is currently known in the art may be as described in U.S. Pat. No. 5,933,224, herein incorporated by reference in its entirety. Although several techniques have been successfully used to measure the time-of-flight, the techniques are generally only as good as their ability to determine the start time at the transmission of the laser pulse and the stop time of the received pulse.
Because the dynamic range of the amplitude of the return pulse is a function of distance, atmosphere attenuation and target reflectivity, a leading edge discriminator using a fixed reference level computer is subject to a walk error Δt as defined by Pasi Palojarvi in his dissertation entitled “Integrated Electronic and Optoelectronic Circuits and Devices for Pulses Time-of-Flight Laser Rangefinding.” (Oulu 2003).
It is known in the art to implement a single comparator in which the reference level is set above the noise floor, and wherein the comparator goes from high to low logic output when the return pulse is above the reference level. For very strong return pulses such as from a retro-reflector, the leading edge of the return pulse may be very close to the actual return pulse time so that the walk error is minimal. However, for weaker signals the leading edge may be extended to several tens of nanoseconds resulting in a range error of several yards. Increasing the fixed gain for weak signals causes saturation of the photodiode circuits and unacceptable signal-to-noise ratios for selected targets.
An additional problem may further present itself in hand-held laser time-of-flight instruments having a fixed gain and a fixed comparator trip level. With the fixed maximum gain, the laser trigger generates a pulse through the photodiode and trans-impedance amplifier circuits that masks the actual return laser pulse from a nearby target. A proprietary circuit used by the Opti-Logic Corporation has fixed comparator trip levels and a time control gain (or “TGC”). When the initial sequence begins for measuring the distance to a target, the electronic gain is minimal but increases with time over multiple laser transit pulses until a return pulse is detected or the maximum gain is reached. If a return pulse is detected, the TGC is reset to the minimum and the increasing gain sequence repeats. This process continues until enough data are obtained from the return pulses to allow the microprocessor to generate and display a valid distance.
However, because the signal level at which the TGC reaches a valid distance may not yield the optimum leading edge for the return pulse, the walk error may be worse than if the gain were always at a maximum.
Therefore, it would be desirable to provide circuitry and methods for accurately and reliably determining the leading edge of a laser return pulse.